In telephone installations the subscriber sets are connected to the exchange by so-called subscriber lines, which are terminated in the exchange at an adaption circuit, or subscriber line interface circuit (SLIC). Such a circuit is used, inter alia, to supply DC current to the associated subscriber apparatus.
A fixed DC voltage may be utilized for this current supply in the prior art. In such a case, the current flowing through the subscriber line will be a function of the internal resistance of the subscriber apparatus or station, and of course the line length. The internal resistance in the apparatus can be kept within narrow limits, but the line length and line resistance may vary considerably. If there is a fixed DC voltage, this must be set to a value such that sufficient DC current can be fed out on long lines also. Thus a larger current than necessary flows through short lines, and it is necessary to reckon with larger losses than necessary in both line and adaption circuit, above all in the supply resistances in this circuit.
The heat due to losses puts a limit on the packing desity of the circuits in the exchange, and it is therefore a primary object to reduce the generation of heat.
It is similarly already known to simulate the physical supply resistances with the aid of feed-back amplifiers, thereby using low-ohmic resistance for measuring and protective purposes. In this way power generation in the supply resistances can be reduced, but the corresponding power is instead generated in the output stage of the amplifiers. Power generation in the output stage is greatest for short lines, and is determined by the outgoing line current multiplied by the DC voltage excess occurring across the amplifier stage.
In order to reduce the power generation in the amplifiers in the next stage it is known, in principle, to take up the excess voltage at no power in the DC/DC converter, the total power generation in the line circuit thus being considerably reduced.
Two principles are described in the litterature for the impedance simulation, c.f. PROCEEDINGS OF THE IEEE, vol. 68, NO 8, AUGUST 1980, pages 991-1009, for example. According to one principle, the line current is sensed to form an instantaneous value in a control system controlling the line voltage. This method gives simple conditions for the operating point setting of the participating amplifiers and control of DC/DC converters. However, the method has the considerable disadvantage that the amplifier which has to sense the line current out on the line will be difficult to achieve with sufficient precision.
According to the other method, the line voltage is sensed and the control system controls the line current. This method gives more simple conditions for realizing the sensing amplifier, but on the other hand it gives difficulties in setting the operating points of the amplifier and control of DC/DC converters.
The most obvious way of realizing a line circuit, per se developing low power in accordance with the above-mentioned principles, is to arrange a DC/DC converter, e.g. in the form of a so-called chopper in the current supply path to each speech wire. The necessary AC symmetry (grounding balance) in the speech frequency band is thus retained right down to DC. However, from the aspect of saving in components, there is preferred an arrangement with only one chopper, thereby accepting the lack of symmetry occurring at least for DC and low frequencies. The symmetry demands for the speech band are met, however. When such a solution with only one chopper is used, the difficulties mentioned above with the operating point setting and control of the chopper are greater than when using the other impedance simulation method according to the above.